Autodeposition has been in commercial use on steel for about thirty years and is now well established for that use. For details, see for example, U.S. Pat. Nos. 3,063,877; 3,585,084; 3,592,699; 3,674,567; 3,791,431; 3,795,546; 4,030,945; 4,108,817; 4,178,400; 4,186,226; 4,242,379; 4,234,704; 4,636,264; 4,636,265; 4,800,106; and 5,342,694. The disclosures of all these patents are hereby incorporated by reference. Epoxy resin-based autodeposition coating systems are described in U.S. Pat. No. 4,180,603 (Howell. Jr.); U.S. Pat. No. 4,289,826 (Howell Jr.); U.S. Pat. No. 5,500,460 (Ahmed et al.); and Internati Publication Number WO 00/71337, the teachings of each of which are incorporated by reference.
Autodeposition compositions are usually in the form of a liquid, usually aqueous solutions, emulsions or dispersions in which active metal surfaces of inserted objects are coated with an adherent resin or polymer film that increases in thickness the longer the metal remains in the bath, even though the liquid is stable for a long time against spontaneous precipitation or flocculation of any resin or polymer, in the absence of contact with the active metal. When used in the autodeposition process, the composition when cured forms a polymeric coating. “Active metal” is defined as metal that spontaneously begins to dissolve at a substantial rate when introduced into the liquid solution or dispersion. Such compositions, and processes of forming a coating on a metal surface using such compositions, are commonly denoted in the art, and in this specification, as “autodeposition” or “autodepositing” compositions, dispersions, emulsions, suspensions, baths, solutions, processes, methods or a like term. Autodeposition is often contrasted with electrodeposition. Although each can produce adherent films with similar performance characteristics, the dispersions from which they are produced and the mechanism by which they deposit are distinctly different. Electrodeposition requires that metal or other objects to be coated be connected to a source of direct current electricity for coating to occur. No such external electric current is used in autodeposition.
Prior epoxy dispersions used for autophoretic application are generally made by preparing low solids solutions of advanced epoxy resin in organic solvent(s). The epoxy resin in organic solvent(s) may be blended with other coating components and additives. The resulting mixture is dispersed in water with surfactant and is then mechanically dispersed to a desired particle size. Because a high concentration of solvent is undesirable in an autodepositing coating bath, these solvent-rich emulsions require removal of solvent by distillation techniques known in the art. The dispersion may then be used as a component of a coating bath. The dispersion, when autodeposited on a metal surface and cured, forms a polymeric coating.
One important factor contributing to the corrosion protection for metal subtrates is the moisture or water barrier properties of the polymeric coating. This is particularly true for coatings applications such as for example those utilized for automotive frames. Prior coatings have been utilized which are based on immersion of such parts in mixtures comprising melted wax. Although these coatings do provide a high degree of corrosion resistance stemming from their water barrier properties, significant shortcomings exist particularly with respect to their heat resistance. Such non-crosslinked, low melting coatings flow away from the parts upon heating leaving the parts unprotected against corrosion. The present invention provides tough, heat and chemical resistant coatings that afford corrosion protection over the lifetime of the part.
Thus, there is a need in the art for high solids, epoxy-based autodeposition coating materials with improved moisture barrier and/or anti-corrosive properties that also eliminate and/or minimize the need for process solvents, which adversely affect manufacturing costs. This need is especially pronounced in thermoset epoxy-based autodeposition formulations utilizing crosslinking agents where process solvent requirements are greatest. There is also a need in the art for epoxy-based thermosetting coatings with improved moisture barrier and/or anti-corrosive properties, which provide high degrees of film coverage over thin edges of the substrate following cure. Shortcomings of the prior art stem from flow away from substrate edges as viscosity of the applied film is reduced as the film is heated in the curing process. There is also a need in the art for a process to obtain these high solids, epoxy-based autodepositing coating materials with improved moisture barrier and/or anti-corrosive properties. Finally, there is also a need in the art for high solids, epoxy-based autodeposition coating materials that are improved with respect to edge protection, mechanical, adhesive and moisture barrier and/or anti-corrosive properties.